The present invention relates to a pressure pad, particularly for magnetic tapes.
As known, in magnetic tape recording and playback systems, such as audio, video or digital systems, the magnetic tape contacts fixed or rotating recording and playback heads and/or other fixed or movable elements, such as tape pressure pads and tape cleaning pads.
Consequently, pressure pads are required such as, e.g., leaf spring type pressure pads, tape-cleaning pressure pads etc., which are capable of ensuring a long working life of the magnetic tape, without causing wear of the tape and without causing any damage to the recording and playback heads.
Such presser pads must meet several requirements; they should keep the magnetic tape free from dust, have a low coefficient of friction, good anti-static characteristics, and be capable of assuming various configurations, without thereby creating localized pressure variations on the magnetic tape.
Pressure pads for magnetic tapes must also avoid creating any constriction in the normal translatory sliding movement of the magnetic tape.
Such pressure pads are also required to inhibit the transport of any fibers which may come loose due to wear of the pressure pad.
The current state of the art provides various types of low cost pressure pads which generally meet one of the above mentioned requirements.
Known pressure pads usually have a pad that is made of synthetic fibers so as to be relatively compact. However, pads made of pure wool or rabbit hair are also known.
This known type of pad made of synthetic fibers has the advantage of having a good coefficient of friction (K) equal to 0.15-0.18, and good dust confinement, which can be estimated at 7-8 mm.sup.3 /cm.sup.2.
On the other hand, known pressure pads have long fibers which are susceptible to breakage and imply the use of extremely expensive materials.
Another disadvantage of the known pressure pads resides in the fact that the fibers are arranged at random on the pad, which can cause incorrect sliding of the magnetic tape.
Another known solution employs a base made of foamed polyurethane and applied to a leaf spring or to a tape-cleaning pad. A layer of material is then applied on said base for sliding contact engagement with the magnetic tape.
The use of a foamed polyurethane base provides good adaptability to the different profiles of the magnetic tape heads. However, the inherent elasticity of the base varies excessively when temperature variations occur, and the foamed polyurethane has a certain tendency to hydrolyze.
The foamed polyurethane base has a very low cost. A layer of material having a low coefficient of friction is applied onto the base. Such layer is often constituted by a polyester film which may have a graphite coating. Thus, a low coefficient of friction is achieved but no dust confinement is possible.
A nonwoven fabric is also sometimes constitutes the layer of material applied onto the base. Such fabric has a relatively low dust confinement, assessable at 2-3 mm.sup.3 /cm.sup.2, and a relatively coefficient of friction (K=0.2-0.22).
The layer applied onto the base is also sometimes made of conductive teflon, which has a low coefficient of friction (K=0.15-0.18) and good antistatic characteristics. However, teflon does not allow dust confinement and is relatively expensive.
In magnetic tape pressure pads of the type adapted for cleaning the magnetic tape, pads capable of discharging static electricity are used. Such pads are constituted by a film of plastic material, which may be conductive or rendered conductive by application of a conductive material, such as graphite or teflon. These pads achieve a low coefficient of friction (K=0.15-0.18), but they are relatively expensive and do not allow dust confinement.
Another known solution resides in the manufacture of a synthetic pad. This solution is frequently used in magnetic tape pressure pads for audio cassettes. The synthetic pad has elastic qualities similar to wool, a very low loss of fibers, an acceptable coefficient of friction (K is approximately equal to 0.2), good dust confinement (with a value of approximately 5-6 mm.sup.3 /cm.sup.2), and a relatively low cost.
Thus, while the various known types of pressure pads are advantageous from some points of view, they are not devoid of disadvantages. Therefore, the choice of the type of pressure pad depends in practice upon which advantageous aspect of the various known pressure pads is preferred.